1. Field of the Invention
The present invention relates to a semiconductor device and a fabrication method therefor, and, in particular, to a semiconductor device that is capable of miniaturization and has a contact structure using aluminum, and to a fabrication method therefor.
2. Description of Related Art
In a semiconductor device such as an LSI, recent advances in miniaturization, integration, and multi-layering of electronic elements have made it necessary to form contact holes with large aspect ratios. An important technical problem that has recently started to cause concern is the difficulty of filling such contact holes with a wiring material. Attempts have been made to fill contact holes with aluminum or an aluminum alloy, which is useful as a wiring material.
One of these techniques is disclosed in Japanese Patent Application Laid-Open No. 64-7673o, by way of example. This discloses a fabrication method in which aluminum is made to fill contact holes in a two-step manner, by a technique of first depositing aluminum or an aluminum alloy at a temperature of 150xc2x0 C. or less, then further increasing the layer of the aluminum or aluminum alloy by bias sputtering.
With this technique, the aluminum of the first layer can be deposited comparatively uniformly and the coverability thereof is improved somewhat, but it is not improved far enough to solve the problem of breakdown of the conductive portions within the contact holes due to causes such as voids.
An objective of the present invention is use aluminum or an aluminum alloy as a conductive substance within contact holes, to provide a semiconductor device having a contact structure with superlative step coverage, without any voids or broken wiring.
Another objective of the present invention is to provide a method of fabricating such a semiconductor device.
The method of fabricating a semiconductor device of the present invention comprises the following steps (a) to (f);
(a) a step of forming a contact hole in an interlayer dielectric formed on a semiconductor substrate including an electronic element;
(b) a degassing step for removing gaseous components included within the interlayer dielectric, by thermal processing under a reduced pressure at the substrate temperature of 300xc2x0 C. to 550xc2x0 C.,
(c) a step of forming a barrier layer on the interlayer dielectric and the contact hole;
(d) a step of cooling the substrate to a temperature of no more than 100xc2x0 C.;
(e) a step of forming a first aluminum layer on the barrier layer, at a temperature of no more than 200xc2x0 C., including one of aluminum and an alloy in which aluminum is the main component; and
(f) a step of forming a second aluminum layer on the first aluminum layer, at a temperature of at least 300xc2x0 C. including at least one of aluminum and an alloy in which aluminum is the main component.
One characteristic of this method of fabricating a semiconductor device is the inclusion within step (b) of a step of removing any gaseous components (the degassing step) that may be comprised within the interlayer dielectric, under special conditions. The inclusion of this degassing step makes it possible to suppress the generation of gases such as water, nitrogen, hydrogen, or oxygen that may be comprised within the interlayer dielectric, during subsequent steps such as the formation of the second aluminum layer under high-temperature conditions of 300xc2x0 C. or more.
The present inventors have confirmed that gases generated from the interlayer dielectric in such a manner would be adsorbed by the barrier layer but would not be adsorbed by the aluminum layers within contact holes. Therefore, the removal of any gaseous components comprised within the interlayer dielectric in step (b) ensures that deterioration in the wettability of the barrier layer and the generation of voids can be reliably suppressed which are caused by gases lying between the barrier layer and the first aluminum layer. As a result it is possible to form contact portions of low-resistance aluminum within the contact holes, with good coverage.
In this case, xe2x80x9cgaseous componentsxe2x80x9d refers to gases that are generated from the deposited layers, that is, the interlayer dielectric and the barrier layer, during conditions of a reduced pressure and a substrate temperature of 300xc2x0 C. or more, such as water, hydrogen, oxygen, and nitrogen. In addition, xe2x80x9cunder a reduced pressurexe2x80x9d refers to a pressure that is preferably no more than 2.6 Pa; more preferably no more than 1.3 Pa.
With the method of the present invention, the temperature of the substrate is cooled to below 100xc2x0 C. by this step (d); preferably to between room temperature and 50xc2x0 C. This cooling of the substrate temperature in step (d) makes it possible to lower the substrate temperature sufficiently before the first aluminum layer is formed. Since the degassing of step (b) is performed at a high substrate temperature of 300xc2x0 C. or more, lowering the substrate temperature reliably in step (d) ensures that the temperature can be adjusted reliably for the subsequent step (e). The inclusion of this step (d) makes it possible to greatly reduce the amount of gases emitted from the interlayer dielectric, the barrier layer, and also all the surfaces of the wafer during the formation of the first aluminum layer. As a result, it is possible to avoid the effects of harmful gases, which are adsorbed to the boundary surface between the barrier layer and the first aluminum layer, on coverability and adhesiveness.
Forming the first aluminum layer on the barrier layer in step (e) at a temperature of no more than 200xc2x0 C., preferably 30xc2x0 C. to 100xc2x0 C., makes it possible to suppress the emission of gaseous components comprised within the interlayer dielectric and the barrier layer, thus making it possible to prevent any deterioration in the wettability of the barrier layer due to the generation of gases from the barrier layer to outside. As a result, the first aluminum layer can be well adhered to the barrier layer, enabling film formation with good step coverage.
The presence of this first aluminum layer makes it possible to suppress the generation of gases from the interlayer dielectric and the barrier layer that underlie the first aluminum layer, even when the temperature of the substrate rises. As a result, the step (f) of forming the second aluminum layer can be performed at a comparatively high temperature, that is, at a temperature high enough for the aluminum or aluminum alloy to flow and diffuse. More specifically, this second aluminum layer can be formed at a temperature of 300xc2x0 C. or more; preferably 350xc2x0 C. to 450xc2x0 C.
In this manner, it is possible to fill the contact holes with good step coverage but without any voids, by forming the first aluminum layer at a comparatively low temperature in step (e) then forming the second aluminum layer at a comparatively high temperature in step (f). It has also been confirmed that the fabrication method of the present invention can be applied to contact holes of a diameter of 0.2 xcexcm.
It is preferable that a layer, called a wetting layer, is not formed on the surface of the barrier layer. A wetting layer is formed over the surface of the barrier layer to increase its wettability with respect to the conductive substance, for example, when a conductive substance is to fill narrow contact holes that have a diameter of 0.5 xcexcm or less and an aspect ratio of 1 to 4. This is usually formed of a film of a refractory metal, such as titanium. However, the present inventors have confirmed that a film of a metal such as titanium is more likely to comprise water or hydrogen. Therefore, if no such wetting layer is formed on the surface of the barrier layer, the quantity of gaseous components will be less than when there is such a wetting layer, making it possible to suppress the generation of gases that cause voids.
The formation of aluminum layers in steps (e) and (f) is preferably done by a sputtering, and it is further preferable that the first aluminum layer and the second aluminum layer are formed in sequence within the same chamber. Forming the aluminum layers in sequence in the same chamber in this manner facilitates control over the substrate temperature and also enables accurate control over the environment, thus making it possible to avoid problems such as the formation of oxides film on the surface of the first aluminum layer.
Steps (d), (e), and (f) are preferably performed sequentially within the same apparatus having a plurality of chambers and maintaining a reduced-pressure state. This makes it possible to reduce the number of substrate movement and placement steps, which makes it possible to simplify the process and prevent contamination of the substrate.
It is further preferable that the barrier capabilities of the barrier layer are improved by introducing oxygen into the barrier layer after the step of forming the barrier layer in step (c), so as to form oxides of the metal of this barrier layer, in parts of the barrier layer. Methods of introducing oxygen into the barrier layer that could be used include exposing the substrate to an oxygen plasma, or subjecting it to thermal processing in an oxygen environment.
A semiconductor device fabricated by the above fabrication method comprises:
an interlayer dielectric formed on the semiconductor substrate from which gaseous components have been removed by thermal processing;
a contact hole formed in the interlayer dielectric;
a barrier layer formed on the interlayer dielectric and the contact hole; and
an aluminum layer formed on the barrier layer and including one of aluminum and an alloy in which aluminum is the main component.
This semiconductor device is characterized in having an interlayer dielectric which has had gaseous components removed therefrom by thermal processing, and it has a contact portion formed of aluminum, layers with good step coverage, as described above.
The contact structure of the present invention could be applied as appropriate to a silicide layer formed on the surface of an impurity diffusion layer that configures a source or drain region of a MOS element, but the present invention is not limited thereto and it could equally well be applied to contact with another region on an impurity diffusion layer having no silicide layer.
The contact hole in accordance with the present invention could be formed by anisotropic dry etching, or it could equally well be applied to a configuration in which an upper end portion of a contact hole is formed to have a moderately tapered shape by a combination of isotropic wet etching and anisotropic dry etching. This would be extremely useful because it would make it possible to use a general-purpose sputtering apparatus that does not have high-temperature resources to enable the formation of the second aluminum layer at 300xc2x0 C. to 350xc2x0 C., when a contact hole of this type is formed in such a manner that a lower portion thereof is formed by anisotropic dry etching to a diameter of 0.5 to 0.8 xcexcm, with an aspect ratio of between 0.5 and 3.0